The present invention relates generally to a method of recording a plurality of images in superposition on a recording medium.
A recording apparatus as shown in FIG. 7 has been proposed in the copending U.S. patent application Ser. No. 07/175,392 filed by Masashi UEDA et al on March 30, 1988. The recording apparatus in FIG. 7 includes a monochromatic laser printer 1 in which a scanner unit 2 is provided. The scanner unit 2 includes a multi-face polygon scanner 2a rotatably provided for directing a laser light onto a photosensitive drum 3. The drum 3 has been previously charged to a predetermined polarity and when the laser light is irradiated thereonto, an electrostatic latent image is formed thereon. The latent image on the drum 3 is developed by toner powders contained in a toner reservoir 4, and the thus developed toner image is transferred onto an ordinary paper or a transparent sheet for use, for example, in an overhead projector, which is fed from a sheet cassette 5. The toner image transferred onto the paper or sheet is then fixed by a fixing unit 6. Normally, the monochromatic laser printer 1 records data transmitted from a host computer (not shown). A color image recording apparatus 20 is disposed beneath the laser printer 1 and records an image using mask members having a light shielding image thereon recorded in accordance with the color picture data. In the apparatus shown in FIG. 7, when a monochromatic image output is needed, such a copy is produced by the monochromatic laser printer 1 and is discharged onto a tray 11 or 12.
The color recording apparatus 20 includes a paper guide unit 10 which guides the mask members 22R, 22G and 22B to be fed into an exposue init 36. An exposure stand 25 is disposed beneath the exposure unit 36, which brings the mask member to facial contact with a photosensitive pressure-sensitive recording medium 24. In the exposure unit 36, light sources 21R, 21G, 21B are arranged in a direction perpendicular to the sheet of drawing for emitting three primary color lights of red, green and blue, respectively. In the drawing, only one light source is illustrated for the sake of simplicity. The recording medium 24 is sequentially exposed to light from each of the light sources through the associated mask member, and is then fed into a developing unit 28. A sheet cassette 27 is provided in the bottom portion of the apparatus for accommodating a stack of developer sheets 26. One surface of the developer sheet 26 is coated with a developer material which reacts with a chromagenic material encapsulated in a microcapsule on the surface of the recording medium 24. The developer sheet 26 and the recording medium 24 are fed into the pressure developing unit 28 while being held in facial contact with each other, in which the microcapsules which remain uncured are ruptured and an image is developed on the developer sheet 26. After passing through the developing unit 28, the exhausted microcapsule sheet 24 is wound around a take-up roller 37 and the developer sheet 26 is fed into a thermal fixing unit 29 for accelerating the fixing of the developed image. A discharge tray 30 is provided for receiving the output copy sheet. An insertion tray 31 is provided for allowing the mask members to be inserted therefrom. The mask members are discharged onto a discharge tray 32. An insertion tray 33 is further provided for allowing a developer sheet to be inserted therefrom.
The apparatus shown in FIG. 7 uses a photosensitive pressure-sensitive recording medium as disclosed, for example, in U.S. Pat. Nos. 4,440,846 or 4,399,209. The former U.S. Patent discloses a self-contained type recording medium and the latter discloses a transfer type recording medium. In the self-contained type, an encapsulated chromogenic material or dye precursour and a developer material are co-deposited on one surface of a single substrate as one layer or as two contiguous layers. In the transfer type, the developer material is coated on a separate substrate as a separate developer sheet. Both the self-contained type and the transfer type are sensitive to light and pressure.
More specifically, such a photosensitive pressure-sensitive recording medium has a substrate on which three kinds of pressure rupturable microcapsules are dispersely deposited. Cyan (C), magenta (M) and yellow (Y) chromogenic materials or dye precursours are separately encapsulated in the microcapsules together with a photo-curing or photo-softening material. The three kinds of the microcapsules are different in photo-sensitivity depending upon the wavelength of light. For example, the microcapsules containing cyan, magenta and yellow chromogenic materials are photo-cured or photo-softened in response to the wavelengths of 650 nm, 550 nm, 450 nm, respectively. By exposing the recording medium to the lights of such wavelengths, a latent image is formed thereon. The recording medium is then subjected to pressure development to repture the microcapsules which have not photo-cured or remained softened and to react the chromogenic materials released from the ruptured microcapsules with a developer material, whereby a visible image is provided on the same recording medium or a separate developer sheet.
Referring to FIG. 2, a priciple of color image reproduction will be described in far more detail. In the following description, a recording medium of the transfer type and photo-curing type will be taken as an example. When the recording medium is exposed to light from a red-color light source 21R through a mask member 22R, the exposed cyan microcapsules are photo-cured whereas unexposed cyan microcapsules remain uncured. The magenta and yellow microcapsules are not photo-cured even if those are exposed to the red-color light. Similarly, the magenta and yellow microcapsules are photo-cured when exposed to green and blue lights, respectively. In this manner, the same recording medium is successively exposed to the respective lights through the associated mask members 22R, 22G and 22B. In a portion of the recording medium to which the lights are applied because of absence of mask portions in any of the mask members, all the microcapsules therein are photo-cured, so that this portion is reproduced in white (W) color. The portion shielded by the mask member 22R is reproduced in cyan (C) color, since the cyan microcapsules remain uncured. Likewise, the portion shielded by the mask member 22G is reproduced in magent (M) color and the portion shieldied by the mask member 22B is reproduced in yellow (Y). The portion shielded by both the mask members 22R and 22G is reproduced in blue (B), the portion shielded by both the mask members 22G and 22B is reproduced in red (R), and the protion shielded by both the mask members 22B and 22R is reproduced in green (G). The portion which has not exposed to any of the lights is reproduced in black (K). According to such a principle, a desired color image can be reproduced by the use of three kinds of the mask members.
However, the use of the three mask members is liable to incure positional displacement of the images when the mask image on each of the mask members is formed in superposition on the recording medium. Therefore, when the exposure is taken placed, it is requisite that the positioning of each of the mask members be ensured, otherwise color displacement or color slip would result. Such a color slip is particularly notable when reproducing a character of black color.
FIG. 4 is an explanatory diagram for description of a character reproduction. In FIG. 4, denoted by 101R, 101G and 101B indicate the same character on each of the mask members. If the mask members are slightly displaced with respect to a stationarily positioned photosensitive pressure-sensitive recording medium when exposure is carried out, the overlapped poriton 102 indicated by oblique lines is reproduced in black but the regions adjacent thereto are reproduced in colors of cyan (C), magenta (M), yellow (Y), red (R), blue (B), green (G), or the like. As indicated, the character as reproduced is blurred and is not good in resolution power.
Although it has been described a conventional recoding method in which a plurality of mask members are used for recording an image on a single sheet of original. It is desirable that a plurality of images on different originals be recorded in superposition on a single image recording medium.